/Engine/Source/Runtime/GeometryCore/Public/DynamicMeshEditor.h
Find "new" vertex for input vertex underIndexmapping, or create new if missing
index of duplicate vertex
int FindOrCreateDuplicateVertex ( int VertexID, [FMeshIndexMappings](API\Runtime\GeometryCore\DynamicMesh\FMeshIndexMappings) & IndexMaps, [FDynamicMeshEditResult](API\Runtime\GeometryCore\FDynamicMeshEditResult) & ResultOut )